lightdm module: Extract greeter configuration and add theme options

[TimoMeijer]

I've extracted the configuration for the lightdm-gtk-greeter, to potentially allow for future greeters, and making configuring them more structured.

I've also added options to being able to configure the theme of this greeter, without reimplementing the whole greeter wrapper.

This is my first pull request to this repository, so any feedback is welcome.

[mention-bot]

By analyzing the blame information on this pull request, we identified @wkennington, @ocharles and @viric to be potential reviewers

[ocharles]

Unused

[ocharles]

This is looking great! Just a quick question and one suggestion (removing an unused variable) then we're good to go.

[TimoMeijer]

@ocharles I've pushed the fix for the unused variable and an unwanted tab, and added an explanation to your suggestion, as I believe it should work this way.

[TimoMeijer]

@ocharles I have pushed a final version after getting some help from @lethalman. It improves the greeter option and should fix the issues with multiple greeters. The current behaviour is:

• A user can enable another greeter(When they get added)
• A user can override the greeter options to set a custom greeter
• When multiple greeters are enabled, it should error

[ocharles]

Excellent, looks great to me! Thanks :)

[the-kenny]

This PR breaks evaluation if you don't use LightDM:

building the system configuration...
error: while evaluating the attribute ‘buildCommand’ of the derivation ‘nixos-system-moritz-x230-16.03.git.0b7fd5f’ at /home/moritz/dev/nixpkgs/nixos/modules/system/activation/top-level.nix:102:7:
while evaluating the attribute ‘sources’ of the derivation ‘etc’ at /home/moritz/dev/nixpkgs/nixos/modules/system/etc/etc.nix:12:5:
while evaluating anonymous function at /home/moritz/dev/nixpkgs/nixos/modules/system/etc/etc.nix:20:20, called from undefined position:
while evaluating the attribute ‘source’ at /home/moritz/dev/nixpkgs/lib/attrsets.nix:154:44:
while evaluating anonymous function at /home/moritz/dev/nixpkgs/lib/modules.nix:74:45, called from /home/moritz/dev/nixpkgs/lib/attrsets.nix:154:52:
while evaluating the attribute ‘value’ at /home/moritz/dev/nixpkgs/lib/modules.nix:287:9:
while evaluating the option `environment.etc.lightdm/lightdm-gtk-greeter.conf.source':
while evaluating the attribute ‘mergedValue’ at /home/moritz/dev/nixpkgs/lib/modules.nix:314:5:
while evaluating anonymous function at /home/moritz/dev/nixpkgs/lib/modules.nix:314:32, called from /home/moritz/dev/nixpkgs/lib/modules.nix:314:19:
while evaluating ‘check’ at /home/moritz/dev/nixpkgs/lib/types.nix:106:15, called from /home/moritz/dev/nixpkgs/lib/modules.nix:315:10:
while evaluating the attribute ‘text’ of the derivation ‘lightdm-gtk-greeter.conf’ at /home/moritz/dev/nixpkgs/pkgs/build-support/trivial-builders.nix:10:14:
while evaluating the attribute ‘background’ at /home/moritz/dev/nixpkgs/lib/attrsets.nix:154:44:
while evaluating anonymous function at /home/moritz/dev/nixpkgs/lib/modules.nix:74:45, called from /home/moritz/dev/nixpkgs/lib/attrsets.nix:154:52:
while evaluating the attribute ‘value’ at /home/moritz/dev/nixpkgs/lib/modules.nix:287:9:
while evaluating the option `services.xserver.displayManager.lightdm.background':
The option `services.xserver.displayManager.lightdm.background' is used but not defined